Scientists put carbon ceiling at a trillion tonnes

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Scientists hope a new approach to assessing carbon build-up in the atmosphere will simplify issues for policymakers and economists.

Two papers published in Nature today (29 April) show that the timings of carbon emissions are not relevant to the debate — it is the total amount of carbon dioxide emitted over hundreds of years that is the key issue.

Rather than basing negotiations on short-term goals such as emission rates by a given year, the researchers say the atmosphere can be regarded as a tank of finite size which we must not overfill if we want to avoid a dangerous temperature rise.

Climate policy has traditionally concentrated on cutting emission rates by a given year, such as 2020 or 2050, without placing these goals within the overall context of needing to limit cumulative emissions.

Both papers analyse how the world can keep the rise in average surface temperatures down to no more than two degrees Celsius above pre-industrial levels. This figure is widely regarded as the threshold beyond which the risk of dangerous climate change rapidly increases. Policymakers around the world have adopted this limit as a goal.

The first study, led by Myles Allen from the University of Oxford, UK, found that releasing a total of one trillion tonnes of carbon dioxide into the atmosphere between 1750 and 2500 would cause a "most likely" peak warming of two degrees Celsius. Emissions to 2008 have already released half of this.

Allen said in a press briefing this week (27 April): "It took 250 years to burn the first half trillion tonnes and, on current predictions, we'll burn the next half trillion in less than 40 years."

The second study, led by Malte Meinshausen at the Potsdam Institute for Climate Impacts Research, Germany, used a computer model to demonstrate that to avoid exceeding two degrees Celsius by 2100, cumulative carbon emissions must not exceed 0.9 trillion tonnes.

"We have already emitted a third of a trillion in just the past nine years," Meinshausen says.

David Frame, a co-author of the Allen paper and researcher at the University of Oxford, said that these findings make the problem "simpler" than it's often portrayed.

"[The findings] treat these emissions ... as an exhaustible resource. For economists, this way of looking at the problem will be a huge simplification," Frame said.

"Basically, if you burn a tonne of carbon today, then you can't burn it tomorrow … you've got a finite stock. It's like a tank that's emptying far too fast for comfort. If country A burns it, country B can't. It forces everyone to consider the problem as a whole."

In a separate essay, Stephen Schneider of the Woods Institute for the Environment at Stanford University in the United States, discusses what a world with 1,000 parts per million of carbon dioxide in its atmosphere might look like.